The system revolves around the central hub which plugs into your internet connection. The additional sensors for the doors, windows, motion sensors then communicate with the hub. Should one of the sensors go off, a message is sent to Alertme.com which then notifies you and any other specified contacts of what is going on in your home. The only issue we see with this is that we now have to keep track of a bunch of battery operated sensors that would probably end up dying on us.
Read more about it here.